Technical Field
This disclosure relates to an ignition system having an ignition plug.
Related Art
Ignition plugs used as ignition means in a combustion chamber of an internal combustion engine are configured to ignite air-fuel mixture by generating discharge spark at spark discharge gaps between center electrodes and the ground electrodes. The discharge spark is repeatedly generated between the center electrode and the ground electrode, which causes wear on the discharge surfaces and the like. Accordingly, the ignition plug deteriorates, depending on the cumulative usage time.
In an initial stage of spark discharge, charge accumulated in stray capacitance between the center electrode and the surrounding housing flows into the spark discharge gap, and thereby capacitive discharge occurs. The wear of the discharge surfaces become advanced as the capacitive discharge current becomes larger. Accordingly, in order to improve the lifetime of the ignition plug, there is a need for reducing the capacitive discharge current.
In order to reduce the capacitive discharge current, the capacitive current PTL1 (Japanese patent application publication No. 2011-222242) has disclosed a method for reducing the stray capacitance of the ignition plug. In the ignition plug of PTL1, a resistor is disposed between the center electrode and a portion forming the stray capacitance for reducing the stray capacitance of the ignition plug.
However, actually there is manufacturing difficulty involved in filling the space with the resistor at a density which enables sufficient effect of reducing the capacitive discharge current to be obtained as described above. Further, in order to produce the foregoing effect, there is a need for providing the resistor at least near the plug tip. However, in this case, the resistor might deteriorate because of heat stress received from combustion in the combustion chamber, which changes the resistance. As a result, it has been difficult to secure ignitability and reduce the capacitive discharge current, and it has not been possible to improve the lifetime of the ignition plug.